The bulb in the upper right corner (near where the dial light diffuser is on a 600 series model) and the four screw holes under the latch striker, indicating a missing data plate, are "tells" that it probably was a R series (military contract) model.

Terry isn't that a R-520A/URR, the militarized Version of the 600 series Trans-Oceanic? Here is a R-520/URR for comparison:

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Bryant and Cones' The Zenith Trans-Oceanic: The Royalty of Radios 2nd edition is probably the best published source on both versions of the military Trans-Oceanics. The first edition lacked information about the R-520A/URR which emerged after the widespread influence of the internet, the updated 2nd edition details both models.

My set has an interesting provenance: It was owned by the family of one of the founding fathers of the Boy Scouts here in America. He is said to have bought it directly from a Zenith Radio Corp. founder. One of the members of that BSA family owned an antique store here in Virginia. A child would often visit that antique store and admire this T/O (it had a Not for Sale sign on it). When the family was getting ready to close/sell this antique store, the owner gave the T/O to the admiring child.

I found it on my local CL minutes after it was listed. I bought it immediately and picked it up during a major thunderstorm. The "admiring child" and his mother passed the aforementioned story along to me when I picked up the T/O. As far as they knew, the T/O remained as it always was while in the original family. The tag attached to the front has it as an "R-520/URR". Having said all of this, I am going solely by the attached Signal Corp tag information...and the provenance of remaining intact as purchased.

Many of the data plates were removed when the radios were "appropriated" from military service as none of the military Trans-Oceanics were sold to the public. Often the cabinets were also painted black to conceal their identity. Perhaps yours was later mated with the incorrect data plate. The data plate for the R-520A/URR looks like this: